This invention relates generally to high density radio communications systems and more particularly to cellular radiotelephone systems employing digital communications techniques to increase the number of channels available in a fixed radio frequency bandwidth and geographic area.
Radio communications systems which employ controlled transmission and reception parameters to realize a plurality of non-interfering defined coverage areas are well known in the art as cellular radiotelephone systems. Variations in design, direction of radio signal illumination, and techniques of system growth have been the subject of several U.S. Pat. Nos.: 3,663,762--Joel, Jr.--"Mobile Communication System"; 3,819,872--Hamrick--"Mobile Telephone Cellular Switching System"; 3,906,166--Cooper et al.--"Radio Telephone System"; 4,128,740--Graziano--"Antenna Array for a Cellular RF Communications System"; and 4,144,411--Frenkiel--"Cellular Radiotelephone System Structured for Flexible Use of Different Cell Sizes". Cellular systems may further be characterized as being capable of automatically and unobtrusively maintaining radio communications between fixed stations and remote stations as the remote stations move across the borders of the cells.
Some of these traditional systems anticipate an increasing number of users with passing time and have developed graceful methods of subdividing and shrinking cell areas to enable multiple reuse of radio frequencies in a fixed geographic area. Generally, each system grows by making the cells smaller and maintaining the pattern of frequency allocation to each of the cells. There are, however, at least two factors which place a limit on the minimum size to which a cell can be shrunk. These factors are the rate at which remote stations move through the cells, and the non-uniformity of the electromagnetic field in the cell. Both factors relate to the time required to determine the relative location of the remote station and to process a handoff of the remote station from the fixed station of one cell to the fixed station of another cell, where the remote station is currently located.
Determination of the location of a remote station is typically performed by measuring the signal strength or quality of the radio signal as received at the fixed station. Because the electromagnetic field in non-uniform, the measurement of signal strength (or quality) is made a plurality of times or averaged over a period of time. The time required becomes longer as the turbulence of the electromagnetic field increases or as the necessary accuracy of the signal strength measurement increases. Thus, there is a finite amount of time which must be spent in determining the location of the remote station. When the density of remote stations becomes large, dedicated equipment is employed at the fixed stations full time in making signal measurements.
Once the measurement is made, a decision must be made whether a handoff of the remote station to another cell is required. If a handoff is required, one or more candidate cells must be queried for their idle channel status and for a verification of the remote station's signal strength in that candidate cell. Processing of the decision, status, and verification usually requires the intervention of higher level system control functions in addition to the control functions in the serving and candidate cells. Additionally, the remote station must be instructed to tune to a frequency available in the candidate cell and verification of its presence after the handoff must be made by the candidate cell. Thus, a significant amount of time is used for handoff processing.
Digital radio transmission techniques, such as would be used with point-to-point systems, have been considered for high capacity cellular systems but previously have not found practical application due to the cost and complexity of digital equipment required to mitigate the effects of intersymbol interference caused by the multiplicity of signal arrival times at the receivers in the system.